The selective oxidation of carbon monoxide in hydrogen-rich streams has been of considerable technical interest for the purification of reformed hydrogen used in feed gas in ammonia synthesis. Recently, this selective oxidation process, sometimes referred to as preferential oxidation, has attracted interest due to the possibility of using this technology in providing suitable hydrogen fuel for fuel cells. Since carbon monoxide is also oxidized to provide carbon dioxide for carbon dioxide lasers, the use of a catalytic composition, which previously had been found useful in the oxidation of carbon monoxide for use in carbon dioxide lasers, has also been investigated for adaptation for use in providing carbon monoxide-free hydrogen for fuel cell feedstock.
A fuel cell is an electrochemical device that enables converting the chemical energy of fuels directly to electricity. A hydrogen-air polymer electrolyte membrane (PEM) fuel cell stack is currently considered the best means for adapting this technology to most uses. The PEM fuel cell is most efficient using gaseous hydrogen for fuel. Use of a fuel processor to generate a hydrogen-rich feedstock at the point of use eliminates problems of storage and distribution of the hydrogen fuel.
A fuel processor can convert fuels such as alcohol, gasoline, liquid petroleum gas, or natural gas to a hydrogen-rich stream. By a process of steam reforming a stream consisting primarily of hydrogen, carbon dioxide and carbon monoxide can be produced, but the product is generally saturated with water. Processing this stream in a shift reactor reduces the carbon monoxide content to provide relatively more hydrogen by means of the well-known water-gas-shift reaction. This reaction provides a product that contains from 0.2 to 2 percent carbon monoxide by volume, which is sufficient to poison the platinum-based catalytic composition at the PEM anode. It has now been found that, among other possibilities for removing carbon monoxide to the level necessary to prevent poisoning of the PEM catalyst, the same catalytic composition that is used to recombine carbon monoxide and oxygen in carbon dioxide lasers can be used to provide hydrogen feedstock for fuel cells on a level of carbon monoxide removal that is commercially viable. The operating conditions for the processes are essentially different. The removal of carbon monoxide by selective oxidation of a stream containing both carbon monoxide and hydrogen can be accomplished using the same catalytic composition as used in carbon dioxide lasers by controlling an increased oxygen flow to the oxidation process, raising the operating temperature of the oxidation process and avoiding reaction between oxygen and hydrogen as compared to the conditions used to recombine carbon monoxide and oxygen in carbon dioxide lasers.